1. Field of the Invention
The invention relates to a color liquid crystal display (LCD) backlight system for night vision imaging system (NVIS) compatibility, which limits the nighttime emissions from the color display between 450 and 930 nanometers (nm), particularly 630 nm and 930 nm, while maintaining the largest color spectrum possible in the normal daylight mode of operation.
2. Discussion of the Prior Art
NVIS compatibility is frequently required in military aircraft. Night vision compatibility requires that during nighttime operation the intensity of emissions from a color display must be controlled within the spectral range of 450 nm to 930 nm, especially between 630 nm to 930 nm.
The invention provides an apparatus which meets NVIS compatibility for active matrix LCDs while maintaining the largest color gamut possible in a normal daylight mode of operation.
A backlight is necessarily utilized to illuminate for viewing color active matrix LCDs. Avionics applications require high luminance for such displays to be readable under high ambient lighting conditions present during daylight operations and lower luminance for nighttime operations. Typical prior art LCD systems rely on light source dimming and filtering to minimize the emissions therefrom between 630 nm and 930 nm during nighttime operations.
In cockpit displays, daytime luminance requirements range from approximately 100 Foot Lamberts (FL) for transport aircraft to 250 FL for fighter aircraft. Since a typical LCD transmits only 2 to 4.5 percent of the light impinging thereon, daylight luminance requirements for the backlight are very high. In order to produce 100 to 250 FL at 2 to 4.5 percent transmission, the backlight output for daylight operation must be 20 to 50 times as high, 2,000 to 12,500 FL, since only 1/20 to 1/50 of the light is actually transmitted by the LCDs.
In contrast to the high light output requirements for daylight displays, nighttime displays must operate at very low luminance levels; 0.1 FL or lower are frequently required. It will be apparent to those skilled in the art that the dynamic range of the backlight is greater than 2,500 to 1. With the high luminance requirements for daylight operation, it is easy to see that losses in the optical system must be minimized.
Fluorescent lamps are the dominant lighting source for active matrix LCDs. Such lamps exhibit high luminance efficacy and moderate flexibility in packaging. With appropriate geometric design, fluorescent lamps can provide the dynamic range required. Such a lamp, in conjunction with an efficient lighting cavity, can produce a uniform light source suitable for daylight use with an LCD.
Unfortunately, fluorescent lamps have unwanted spectral emissions in the 630 nm to 930 nm range. These emissions come from the phosphors, the arc and the cathodes of the lamp, whether the lamps are hot or cold cathode lamps. Such emission sources are of sufficient intensity to violate the requirements of current military (NVIS) specifications. An additional difficult factor is that although an LCD's transmission is less than 5 percent for visible light, it is nearly 50 percent for light within the 700 nm to 900 nm range.
Some prior art systems provide NVIS compatibility by placing a filter between the fluorescent backlight and the LCD. This filter must absorb or reflect the lamp emissions in the 630 nm to 930 nm range while transmitting light within the visible range. Several disadvantages must be accepted when using total backlight filtering. First, the added optical error of the filter in the light path creates losses due to absorption. This requires more light be generated from the fluorescent source in order to provide the additional feature of NVIS compatibility. Second, the chromaticity of the LCD is limited, because 630 nm is well within the visible red spectrum. In other words, part of the red in the display is lost. Hence, filtering the backlight for NVIS compatibility limits the color in the red area for normal daylight operation so that the red appears to be less saturated or more orange in color. Such NVIS filters typically comprise absorptive glasses or dichroic hot mirrors. Although the absorptive glasses do a fairly good job of absorbing emissions in the 630 nm to 930 nm band, the transition from the transmission to the absorption band is gradual; the transmission to absorption transition region severely penalizing the red transmission.
Dichroic mirrors can alternatively be used and can provide high transmission with sharp cut-off filtering, thereby allowing for better red transmission. However, dichroic mirrors are very angle sensitive. A hot mirror coating may have a cut-off frequency at 630 nm at normal viewing, but will shift to a cut-off frequency of 500 nm when viewed at a 45.degree. angle, providing a filter which removes essentially all the red for a 45.degree. viewing angle.